1. Field of the Invention
This invention relates to a lock-on-after-launch (LOAL) guidance system and, more specifically, to such a guidance system using three dimensional lock-on-after launch (3D.sub.-- LOAL) scene reconstruction.
2. Brief Description of the Prior Art
Much of the prior art relating to LOAL guidance systems relates to variations of the theme of projecting three dimensional (3D) models into their two dimensional (2D) representations and then registering these representations to an image. Such systems are exemplified by Ditzler, W. R., et al., "Multispectral Image Correlation for Air to Ground Targeting," presented at symposium for "Low-Level and Nap of the Earth (N.O.E.) Night Operations, " Rome, Italy, October, 1994.
In the current state of the missile guidance autonomous LOAL algorithms, a painstaking procedure known as "Prebriefing" or "Target Reference Preparation" is required. "Prebriefing" is the act of preparing a description of the target that the algorithm requires in order to locate the target and "prebrief" is the output of prebriefing. In the prebriefing task, the mission planners must follow a rule based approach in which they model a few salient features of the target area which are: (1) unique in shape and (2) visible in contrast to the missile at its preselected lock-on point. Thus, the mission planner must know "geometry" and "signature". Geometry information can be obtained rather easily, but signature further requires identifying the material types of the target objects and factoring in weather (wind, cloud cover, atmosphere, sun angles, etc.) and internal heat sources (heating/cooling, generators, boilers, etc.). Although the main signature question is whether there exists a moderate amount of contrast for certain regions, this can still be extremely time consuming.
Performance level prediction can also be extremely time consuming because extensive Monte Carlo based (randomized) simulations with feedback iteration on the prebriefing must be performed. Once the prebriefing is prepared, the missile is ready for launching.
Although the actual prebriefing model is three dimensional in nature and defined in a world coordinate system, it is only applicable to a limited set of missile approaches and viewing ranges. Once the missile has flown into this "basket" of approach angles and ranges, the three dimensional template is projected into the two dimensional image planes, hidden surfaces are removed and the template is registered to the image. Here, the two dimensional nature of the registration process becomes a limiting factor in the accuracy of aimpoint (the point the missile tries to guide to in the target area) placement. For example, small errors in position or orientation can cause perspective differences in the image that dilute the registration. Fortunately, there is some tolerance to transformation inaccuracies in this method, however this method provides no mathematical mechanism for correcting these transformation errors. For this reason, aimpoints offset from the center of this template must also be kept to short distances because even small rotation errors can result in quite large aimpoint offset errors which degrade overall circular error probable (CEP), the minimum radius around the aimpoint which contains 50 percent of the actual missile impact points. In many cases, multiple templates must be defined for various stages of the mission to overcome these errors and accommodate pixel-on-target constraints due to target size.